Providing a power supply in a cellular telephone (also referred to as a handset) or similar mobile wireless telecommunication device can be challenging. Goals such as minimizing power consumption may compete with others, such as minimizing adverse effects on the radio frequency (RF) spectrum of the transmitter power amplifier. Power supply challenges can be particularly acute in multi-mode devices, such as those that operate in accordance with a selected one of two or more different transmission standards, such as the Code Division Multiple Access (CDMA) standard, the Enhanced Data Rates for GSM Evolution (EDGE) standard, or the General Packet Radio Service (GPRS) standard.
In some RF power amplifier systems, such as those having a Collector Voltage Amplifier Control (COVAC) architecture, the power amplifier power supply includes a linear voltage regulator. The output of an error amplifier within the linear voltage regulator is connected to a control input of a pass device, such as the gate of a field-effect transistor (FET), which acts as a variable resistance between the power supply and the power amplifier. A p-channel FET (PFET), rather than an n-channel FET (NFET), is commonly employed as the pass device, because at low power supply voltages an NFET will not operate unless its gate voltage could be boosted higher than the power supply voltage when the desired regulator output voltage is close to (typically less than one volt) the power supply voltage. Providing the boosted gate voltage is generally not feasible in a battery-operated device such as a mobile wireless telecommunication device.
In other RF power amplifier systems, the RF power amplifier is directly connected to the battery. However, as it has been recognized that under normal usage conditions such devices do not transmit, on average, at peak power, many of today's RF power amplifier systems include a DC-DC converter between the battery and the power amplifier to step the battery voltage down to a level that economizes on power consumption yet still permits operation under essentially all normal usage conditions. However, including a conventional DC-DC converter in some wireless telecommunication devices can present problems. For example, because DC-DC converters are based upon switching circuitry, they can introduce spurious signals into the power supply at the switching frequencies unless measures are taken to isolate sensitive elements, such as the power amplifier. Also, many conventional DC-DC converters include a large inductor external to the power supply chip, which can take up an undesirably large amount of space. Inductor-less DC-DC converters have been developed, in which capacitors are used as the energy storage elements instead of inductors.
It has been suggested to use inductor-less DC-DC converters to supply wireless telecommunication device power amplifiers. Such DC-DC converters have been successfully employed in some CDMA wireless telecommunication devices because the power amplifiers in such devices inherently provide 20-30 dB supply rejection, since the power amplifier is not voltage saturated. (The term “supply rejection” or “power supply rejection” in this context refers to the ratio of signal content present on the power supply relative to the signal content modulated onto the RF carrier.) Additionally, in a CDMA transmitter the DC-DC converter switching frequency spectrum can often be hidden in-band with limited impact to power amplifier performance.
Integrating a DC-DC converter into a multi-mode device, such as one that operates in accordance with both the GPRS and EDGE standards, presents greater challenges. For example, the Third-Generation Partnership Project (3GPP) standard for GPRS requires that power amplifier output power be controllable over a 35 dB dynamic range and includes spectrum specifications that require significant power amplifier isolation at frequencies near the DC-DC converter switching frequencies. It has been recognized that employing a DC-DC converter in a GPRS device power amplifier power supply would require a large linear voltage regulator in series with the DC-DC converter to provide the required frequency rejection in the DC-DC converter, since the frequencies of the spurious switching signals often fall outside the transmit channel. However, a series combination of a switching DC-DC converter plus a linear voltage regulator would unavoidably degrade the peak power performance of the power amplifier. Also, as a PFET pass device would be used with such a linear voltage regulator, providing sufficient voltage regulator frequency rejection while supporting peak power operation would be problematic. The use of a PFET pass device in a GPRS device power amplifier power supply having a series combination of a DC-DC converter and a linear regulator would also limit the minimum DC-DC converter output voltage, due to the gate-to-source voltage requirement of such a PFET device.